Photoelectric potentiometer actuated by position of a light spot



E. M. JONES Sept. 7, 1965 3,205,365

PHoToELEcTRIc .,POTENTIOMETER ACTUATED BY POSITION 0F A LIGHT sIoI1 2 Sheets-Sheet 2 Filed Deo. 28, 1961 @Wm mm Z m! ai VM @Zz m/ e; f y a f u d w ,wm Y?, B u 6 f f United States Patent O 3,205,365 PHOTOELECTRIC POTENTIOMETER ACTUATED BY POSITION F A LIGHT SPOT Edward M. Jones, Cincinnati, Ohio, assignor to D. H. Baldwin Company, a corporation of Ohio Filed Dec. 28, 1961, Ser. No. 162,864 8 Claims. (Cl. Z50-211) The present invention relates to electromechanical transducers, and more particularly to photoelectric position indicating transducers.

Potentiometers, or variable-resistance transducers, are well known devices for generating signals responsive to a shaft position or rectilinear motion. The Control Engineers Handbook published in 1958 by the McGraw-Hill Book Company, Inc., edited by John G. Truxal, describes such potentiometersas resistive elements with a tap that is positioned by the mechanical input. Potentiometers have been used with circular resistance elements in contact with a tap in the form of a wiper mounted on a rotatable shaft, so that such potentiometers may be used to generate a signal responsive to the position of the shaft. Potentiometers also have been used in the form of elongated resistance elements with a wiper mounted for rectilinear motion.

Conventional potentiometers are considered to have the advantage of operating on alternating current with relatively small phase shifts and minor frequency effects.

y They also may be constructed with low impedance, and

may be utilized to generate non-linear functions. Conventional potentiometers have the disadvantage of substantial friction between the wiper or sliding contact and the resistance element. This mechanical movement often results in maintenance problems and generation of electrical noise. Also, because most potentiometers are wire wound, the resolution available with such potentiometers is limited.

It is an object of the present invention to provide a potentiometer suitable for use as an electromechanical transducer which does not have the disadvantages of prior potentiometers, particularly, wiper friction and lack of resolution.

This object of the present invention is achieved by providing a photopotentiometer with a photoconductive film disposed between the resistance element of the potentiometer and the output terminal, and providing means to illuminate only a portion of this photoconductive film at any given time to provide the electrical contact between the resistance element and the output terminal. Variable resistors which operate on a photoconductive principal are not broadly new, Patent No. 1,514,123 to Bacevicz disclosing a resistance element With two spaced bars and a bridge of photoconductive material extending between the bars. In the Bacevicz patent, a mirror is pivotally mounted to reflect light to one portion of the resistance element, and thereby provide electrical contact between the two bars. The Bacevicz device is not a potentiometer, but merely a photoresistor.

It is one of the objects of the present invention to provide a photopotentiometer which is suitable for use as an electromechanical transducer.

It is a further object of the present invention to provide a photopotentiometer which utilizes a film for the resistive element and a film for the photoconductive layer, and further to provide a film type photopotentiometer with a denite and defined center tap.

Additionally, it is an object of the present invention to provide a photopotentiometer with low output impedance, even as compared with conventional photopotentiometers.

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These and further objects of the present invention will become readily apparent from a further consideration of this disclosure, particularly when viewed in the light of the drawings, in which:

FIGURE 1 is a sectional view of the photoresistive element of a photopotentiometer constructed according to the teachings of the present invention;

FIGURE 2 is a sectional view taken along the line 2-2 of FIGURE 1;

FIGURE 3 is a schematic electrical circuit diagram of the photopotentiometer illustrated in FIGURES l and 2;

FIGURE 4 is a partially diagrammatic and partially sectional view illustrating the optical system and resistance element which comprise theV photopotentiometer;

FIGURE 5 is a plan view of the resistance element and optical system which comprise the photopotentiometer;

FIGURE 6 is a fragmentary sectional view illustrating another embodiment of the photoresistive element.

In FIGURES 4 and 5, a photoresistor 10 is illustrated in physical relation with the elements of`an'optical system. The optical system employs a lamp 12 with a filament 14, and a collimating slit structure 16. The lamp 12 and slit structure 16 constitute a source of collimated light, and this collimated beam of light is focused by a cylindrical lens 18 positioned on the side of the slit structure 16 opposite the filament 14. The cylindrical lens 18 which is in the form of a glass rod, forms an image of the filament 14 which is at the focus of a parabolic cylindrical mirror 20. As a result, parallel light rays in the plane of FIGURE 5, which may be considered to be the horizontal plane, are reflected by the parabolic cylindrical mirror 20 to a spindle reflector 22. The spindle reflector 22 is mounted on a shaft 23, and it is the rotational position of this shaft which is to be indicated by the output of the photopotentiometer.

The spindle reflector 22 is also curved, but only in the vertical plane. The spindle reflector 22 directs light impinging thereon against the parabolic cylindrical mirror 20 and when the shaft 23 is positioned to place the spindle reflector 22 perpendicular to the rays from the cylindrical mirror 20, the spindle reflector 22 focuses light reflected through this optical system exactly on the same vertical line at the focus of the parabolic cylindrical mirror 20.

As best illustrated in FIGURE 4, the spindle reflector 22, however, is directed downwardly, so that the light reflected therefrom is reflected from the parabolic cylindrical mirror 20 to impinge upon a composite mirro' 24. The composite mirror 24 directs the light on a prisrr 26 mounted on the photoresistor 10.

The composite mirror 24 has a convex cylindrical central portion 28 in the vertical plane, and concave cylindrical side portions 30 and 32 on opposite sides thereof, also in the vertical plane. Since the image of the filament 14 is also in the vertical plane, the convex central portion 2S acts to magnify this image and to direct its location relative to the center of the mirror. The concave portions 30 and 32 act to limit the translation of the image from the central axis.

FIGURES l and 2 illustrate the photoresistor 10 in detail. The photoresistor has a cylindrical base 34 of electrically insulating material, such as glass. A strip of photoconductive material 36 is disposed along a :straight line on the base 34, and a straight line conductor 38 is disposed centrally on the strip 36 of photoconductive material. A film of resistance material 40 is disposed in a straight line parallel and adjacent to the conductor 38 on the photoconductive strip 36, and a second film 42 of resistance material is disposed in a ystraight strip on the photoconductive Li strip 36 on the opposite side of the conductor 38. It is to be understood that the strip 36, conductor 38 and films 40 and 42 are very thin deposits and have exaggerated thicknesses in the ngures for clarity.

A right angled circular flange 44 is sealed about the perimeter of the base 34, and a cover plate 46 is sealed to the flan-ge 44. It is on this cover plate 46 that the right angled prism 26 is mounted, and the prism 26 is disposed in alignment directly centered above the conductor 38. Since the image `of the filament 14 is in the vertical plane relative to the prism 26, it falls in the horizontal plane on the conductor 38 and resistance films 40 and 42. Since the resistance films 40 and 42 are connected to the conductor 38 through the photoconductive strip 36, light impinging upon the photoconductive strip 36 creates a region of relatively high electrical conductivity between the conductor 38 and the resistance'films 40 and 42.

There are many photoconductive materials which are suitable for the photoconductive films 36. In general, those materials which vare a combination of a cation material from the class consisting of cadmium, lead, indium, mercury, gallium, zinc, and aluminum, with an anion material from the group consisting of sulphur, selenium, tellurium, antimony, and arsenic .are suitable. A particularly suitable composition for the photoconductive film is cadmium selenide. Photoconductive films of the type referred to herein and suitable processes for making such films are set forth in the patent applications of Frances B. Hugle and William B. Hugle, Serial No. 574,804, now Patent No. 2,994,621, and Serial No. 791,400, now Patent No. 3,187,414.

The films 40 and 42 may be constructed of any of the conventional resistive film materials. The articles by S. I. Stein and I. Reiseman entitled Evaporated Metal Film Resistors, Proceedings, 1954 Electronic Component Symposium, Washington, D.C. and the paper The Electrical Properties of Thin Metal Films by I. Reiseman, Transactions of the New York Academy of Sciences, Series II, vol. 19, No. 6, dicloses suitable films. In one construction 'of the present invention, which will be described throughout this specification, the resistance films 40 and 42 are selected to have resistivities of 200 -ohms per square inch. The length .of the films 40 and 42 are 0.36 inch, and the total resistance of the potentiometer is 1400 ohms.

The conductor 38 may be constructed of any one of a number of electrically conducting materials. Inconel, silver, gold, and chromium have been found to be suitable.

As illustrated in FIGURE 2, both ends of the conductor 38 terminate in contact with the flange 44 and the flange 44 thus serves as an output terminal. The film 40 terminates at one end in a terminal strip 48, which extends between a pin 50, and the end of the film 40. The film 40 terminates at the other end in a terminal Strip 52 which extends from a second pin 54 to this end of the film 40. The film 42 also terminates at its ends in terminal strips 56 and 58 which extend between their respective ends of the film 42 and pins 6l) and 62, respectively. The pins 50 and 54, and 60 and 62 extend through the glass base 34 and are hermetically sealed therein. Also, the flange 44 and cover plate 46 are hermetically sealed to each other.

An electrically conducting spear 64 has a pointed end which terminates on the resistive film 40 adjacent to the conductor 38 but spaced therefrom. This end on the spear 64 is a sharp point, and the spear extends outwardly therefrom to terminate in a pin 66. The pin 66 is thus connected to the center tap of the resistor element 40. In like manner, a second spear 68 has a pointed end disposed on the resistive film 42 centrally between the terminal strips 56 and 58. This second spear 68 widens from the point thereof, which is adjacent to the conductor 38, to form an electrical contact with a pin 70. Pin 70 is thus connected to the center tap of resistance element 42. The pins 66 and '70 both extend through the base 34 to form electrical terminals.

FIGURE 3 illustrates the electrical circuit achieved by the photopotentiometer. The resistive film 40 may be considered to be the resistance element 40A, and the resistive film 42 may be considered to be the resistance element 42A. A direct current potential is connected across each of the resistance elements, FIGURE 3 illustrating this direct current potential as plus 5.6 volts to minus 5.6 volts. Also, in FIGURE 3, the center taps formed by the spears 64 and 68 are shown grounded. Asa result of the optical system, the position of the shaft 23 causes an image of the filament 14 of the lamp 12 to fall on the photoresistor, this image being shown at 'l2 in FIGURE 2 and extending from the film 40 to the resistive film 42. As a result, the photoconductive layer 36 yis subjected to light in the region between the film 40 and the conductor 38 and in the region between the film 42 and the conductor 38. Hence a region of illumination of the photoconductive film 36 achieves a `substantially higher electrical conductivity coefficient than the regions of the conductive film which are not illuminated. The photoconductive lm in its illumin-ated condition has a coefficient of electrical conductivity which is substantially greater than the coefiicient of electrical conductivity of the resistive films 40 and 42, but in its dark condition, the photoconductive film has a coefficient of electrical conductivity which is substantially lower than the conductivity of the resistive films 40 and 42. The resistance of the illuminated portion of the photoconductive film is diagrammatically illustrated in FIGURE 3 as resistor 73 and resistor 74, and these are shown as, extending from the film resistance elements 40A and 42A to the electrical conductor designated 38A. The illuminated portions of the photoconductive film, which are diagrammed as the rwistors 73 and 74, thus form an electrical contact between the resistance elements 40 and 42 and the conductor 38A.

The conductor 38, terminal strips 48, 52, 56 and 58, and spears 64 and 68 all have substantially higher electrical conductivity than the resistive lms 40 and 42. Hence the spears 64 and 68, which are illustrated in FIGURE 3 as grounded, in effect ground the center of the resistance films 40 and 42. It is thus apparent that an image of the lamp I2 which is located near the center point, or near theV spears 64 and 68, will produce no output and the center tap of the photoresistor is properly positioned. As the shaft 23 rotates from the zero position, the convex portion 28 of the composite mirror 24 acts to move the image in a definite direction from the center position of the photoresistor 10.

As illustrated in FIGURE 3, the resistors 40A and 42A are center tapped, and the center tap connected to ground. The end terminals of the resistors 40A and 42A are connected to opposite and equal potentials, so that the center tap of each of the resistors is at ground potential and the potential increases by equal linear increments toward one terminal and decreases by equal linear increments toward the other terminal of each resistor. Since the image '72 of the filament 14 of the lamp 12 substantially reduces the resistance of the photoconductive layer 36 between the conductor 38A and each of the resistors 40A and 42A in the region of illumination only, there is in effect a short extending across the photoconductive layer at the position of the image.

The photoresistor is designed to produce a linearly increasing output over a limited range, in one particular construction a range of one degree, but to produce its maximum output for further rotational deviations up to the limit of the transducer. The concave portions 30 and 32 of the composite mirror 24 achieve this result, and in the Particular construction herein described, maintain a relatively constant output for deviation in either direction between 1 degree and 6 degrees.

FIGURE 6 illustratesl a modified photoconductive resistor which may be utilized in the present invention. The optical system described in connection with the photoresistor of FIGURES l and 2, these elements will bear the 4same reference numerals.

The photoresistor of FIGURE 6 has been designated 10A, and it includes the base 34 and flange 44 previously described. The electrically conducting films which are used for the conductor, designated 38B, the spears 64B and 68B, and the terminal strips, all of which correspond to the elements of the previous embodiment, are directly deposited on the flat surface of the base 34. It is to be noted that the electrically conducting strip 38B is spaced from the spears 64B and 68B, as are the terminal strips which have not been illustrated but have the same shape set forth in FIGURE 2. Resistive films 40B and 42B are deposited on the surface of the base 34 and thc elecytrical conductors which have previously been deposited,

these conductors being the terminal strips and spears. The resistive films 40B and 42B do not make a contact with the conductor 38B but terminate at a distance therefrom. A photoconductive film 36B is then disposed over the entire surface of the resistive films 40B and 42B, the conductor 48B, and all regions therebetween. unit is again sealed by a cover plate 46 which carries a prism 26. y

The image of the lamp filament is effective to change the resistance characteristics of photoconductive film 36B, and thereby provide a path of lower resistance between the resistive films 40B and 42B, and the conductor 38B, in the same manner as that previously described in connection with the description of FIGURES 1 and 2.

From the foregoing disclosure, those skilled in the art will readily devise many modified constructions and many other applications for the present invention. It is therefore intended that the scope of this invention be not limited by the foregoing disclosure, but rather only by the appended claims.

The invention claimed is:

1. A photoresistor comprising a base of electrically insulating material having a surface, an elongated electrical conductor having a pair of opposite sides supported by said surface, a first elongated resistance element disposed spaced from and adjacent to one of the sides of the electrical conductor, said first element having a C0- eflicient of electrical conductivity less than that of the conductor, a .second elongated resistance element disposed spaced from and adjacent to the other side of the electrical conductor, said second element having a coefficient of electrical conductivity approximately equal to that of the first resistance element, and a layer of photoconductive material disposed between the conductor and each of the resistance elements, said layer having a coefficient of electrical conductivity no greater than the resistance elements in the dark and greater than said resistance elements when illuminated.

2. A photoresistor comprising a base of electrically insulating material having a flat surface, a straight elongated electrical conductor having parallel opposite sides supported by said surface, a first straight elongated resistance element disposed spaced from and parallel to one of the sides of the electrical conductor, said first element having a coefficient of electrical conductivity less than that of the conductor, a second straight elongated resistance element disposed spaced from and parallel to the other side of the electrical conductor, said second element having a coefficient of electrical conductivity approximately equal to that of the first resistance element, a layer of photoconductive material disposed between the conductor and each of the resistance elements, said layer having a coefficient of electrical conductivity no greater than the resistance elements in the dark and greater than said resistance elem-ents when illuminated, and terminals electrically connected to opposite ends of the resistance elements, and to the electrical conductor.

3. A photoresistor comprising ,a base of electrically insulatingI material having a fiat surface, a straight elongated electrical conductor having parallel opposite sides supported by said surface, a first straight elongated resistance element disposed spaced from and parallel to one The of the sides of the electrical conductor, said first element `having a coefiicient of electrical conductivity less than than said resistance elements when illuminated, two electrically conducting spears, one spear extending across and in contact with one of the resistance elements and terminating at a distance from the conductor and the -other spear extending across and in contact with the other resistance element and terminating at a distance from the conductor, and terminals electrically connected to opposite ends of the resistance elements, to the spears, and to the electrical conductor.

4. A photoresistor comprising a base of electrically insulating material having a surface, an electrical conductor supported by the surface, a resistance element disposed spaced from and adjacent to the electrical conductor, said resistance element having a coefficient of electrical conductivity less than that of the conductor, a layer of photoconductive material disposed between the conductor and the resistance element, said layer having a coefiicient of electrical conductivity no greater than the resistance element in the dark and greater than said resistance element when illuminated, an electrically conducting spear extending across and in contact with the resistance element and terminating at a distance from the conductor, said spear being located between the ends of the resistance element.

S. A photopotentiometer comprising a base of electrically insulating material having a surface, an electrical conductor supported by the surface, a resistance element disposed spaced from and adjacent to the electrical conductor, said resistance element having a coefficient of electrical conductivity less than that of the conductor, a layer of photoconductive material disposed between the conductor and the resistance element, said layer having a coefficient of electrical conductivity no greater than the resistance element in the dark and greater than said resistance element when illuminated, a voltage source electrically connected across the resistance element, a spear of electrically conducting material extending across and in contact with the resistance element and terminating at a distance from the conductor, and means for directing a spot of light onto the photoconductive material, said spot extending between the resistance element and the conductor and covering only a portion of the region between the resistance element and the conductor.

6. A photoresistor comprising a base of electrically insulating material having a surface, a layer of photoconductive material disposed on the surface, an elongated electrical conductor having a pair of opposite sides disposed on said photoconductive layer, a first elongated resistance element disposed on the photoconductive layer spaced from and adjacent to one of the sides of the electrical conductor, said first element having a coefficient of electrical conductivity less than that of the conductor, and a second elongated resistance element disposed on the photoconductive layer spaced from and adjacent to the other side of the electrical conductor, said second element having a coefficient of electrical conductivity approximately equal to that of the first resistance elements, the photoconductive layer having a coefficient of electrical conductivity no greater than the resistance elements in the dark and greater than said resistance elements when illuminated.

7. A photopotentiometer comprising an electrical conductor, an element having a coefficient of electrical conductivity less than the coeilicient of electrical conductivity of the conductor disposed in spaced adjacent relation to the conductor, electrical terminals electrically connected vto the ends of the element, a layer of photoconductive including a mirror having a plano-convex central section and a plano-concave section on each side thereof, the central section being positioned to reflect light on a region of the layer of photoconductive material approximately midway between the ends of the resistance element.

8. A photoresistor comprising the combination of claim 1 wherein the elongated conductor, rst elongated resistance element, and second elongated resistance element are disposed on the surface of the base in adjacent relationship, and the layer of photoconductive material is disposed on the elongated conductor, the rst elongated resistance element, the second elongated resistance element, and the portions of the surface of the base between the elongated conductor and the first and second elongated resistance elements.

References Cited by the Examiner UNlTED STATES PATENTS 1,514,123 11/24 Bacevicz 250-211 X 2,879,405 3/59 Pankove Z50-211 3,028,500 4/ 62 Wallmark 25 0-21 1 3,03 3,073 5 62 Shuttleworth 250-211 X 3,087,069 4/ 63 Moncriet-Yeates 250-211 OTHER REFERENCES Electronics Magazine; vol. 34; No. 32, Aug. 11, 1961, page 178.

RALPH G. NILSON, Primary Examiner.

WALTER STOLWEIN, Examiner. 

1. A PHOTORESISTOR COMPRISING A BASE OF ELECTRICALLY INSULATING MATERIAL HAVING A SURFACE, AN ELONGATED ELECTRICAL CONDUCTOR HAVING A PAIR OF OPPOSITE SIDES SUPPORTED BY SAID SURFACE, A FIRST ELONGATED RESISTANCE ELEMENT DISPOSED SPACED FROM AND ADJACENT TO ONE OF THE SIDES OF THE ELECTRICAL CONDUCTOR, SAID FIRST ELEMENT HAVING A COEFFICIENT OF ELECTRICAL CONDUCTIVITY LESS THAN THAT OF THE CONDUCTOR, A SECOND ELONGATED RESITANCE ELEMENT DISPOSED SPACED FROM AND ADJACENT TO THE OTHER SIDE OF THE ELECTRICAL CONDUCTOR, SAID SECOND ELEMENT HAVING A COEFFICIENT OF ELECTRICAL CONDUCTIVITY APPROXIMATELY EQUAL TO THAT OF THE FIRST RESITANCE ELEMENT, AND A LAYER OF PHOTOCONDUCTIVE MATERIAL DISPOSED BETWEEN THE CONDUCTOR AND 